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Coupling of N-Tosylhydrazones with Terminal Alkynes Catalyzed by Copper(I) Synthesis of Trisubstituted Allenes.

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DOI: 10.1002/ange.201005741
Synthetic Methods
Coupling of N-Tosylhydrazones with Terminal Alkynes Catalyzed by
Copper(I): Synthesis of Trisubstituted Allenes**
Qing Xiao, Ying Xia, Huan Li, Yan Zhang, and Jianbo Wang*
Allenes are uniquely versatile intermediates in organic synthesis because of their structural and reactive properties that,
in many cases, complement the chemistry of alkenes and
alkynes.[1, 2] Allene moieties have also been found in many
natural products as well as pharmaceutically related compounds.[3] Because of the growing importance of this type of
compound, synthetic methods that can rapidly lead to
substituted allenes from simple and readily available starting
materials are highly desirable. Over the past decades,
enormous efforts have been devoted to the development of
highly efficient allene synthesis.[4] Among the various methods hitherto developed, the most generally useful one is based
on SN2’-type displacement of propargyl alcohol derivatives
with organocopper species.[4–6]
With respect to efficiency and versatility, the direct
coupling of two fragments mediated by a transition-metal
catalyst is obviously more attractive to construct the core
structure made of three carbon atoms (Scheme 1). However,
the allene synthesis based on direct coupling is much less
developed and to the best of our knowledge, up until now
there are only three known catalytic methods reported in the
literature. These methods include allene cross-metathesis
(Scheme 1 a), carbene/vinylidene cross-coupling (Scheme 1 b), and the Crabb homologation and its modification
(Scheme 1 c). For the cross-metathesis, so far there is only one
report. Barrett and co-workers employed Grubbs catalyst to
demonstrate that it was possible for one of the terminal
carbon units of allene substrates to be exchanged, thus
affording new symmetrically substituted allenes. A considerable amount of polymer was also formed as side product.[7]
Bertrand and co-workers have recently developed an efficient
AuI complex that catalyzes the coupling of enamines and
terminal alkynes to afford allene derivatives in good yields.[8]
In 1979, Crabb and co-workers reported an allene synthesis
based on a three-component reaction of a terminal alkyne,
[*] Q. Xiao, Y. Xia, H. Li, Dr. Y. Zhang, Prof. Dr. J. Wang
Beijing National Laboratory of Molecular Sciences (BNLMS) and
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of Ministry of Education, College of Chemistry, Peking University
Beijing 100871 (China)
Fax: (+ 86) 10-6275-1708
E-mail: wangjb@pku.edu.cn
Homepage: http://www.chem.pku.edu.cn/physicalorganic/
home.htm
[**] The project was supported by the Natural Science Foundation of
China (Grant Nos. 20902005, 20832002, 20772003, and 20821062)
and the National Basic Research Program of China (973 Program,
No. 2009CB825300).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005741.
1146
Scheme 1. Allene synthesis by direct coupling of two fragments. Ts = 4toluenesulfonyl.
formaldehyde, and diisopropylamine mediated by CuBr.[9a–e]
This reaction, now recognized as Crabb homologation,
results in low yields in many cases.[9] Recently, Kuang and
Ma have significantly improved the Crabb homologation by
replacing CuBr and diisopropylamine with CuI and dicyclohexylamine.[10a]
A severe limitation of the Crabb homologation is that
the reaction only works with formaldehyde, and as a
consequence only monosubstituted allenes can be synthesized
by this method. Very recently, Kuang and Ma disclosed a
breakthrough, which was based on the reaction of aldehydes,
morpholine, and terminal alkynes mediated by ZnI2.[10b] This
new method can be employed to synthesize 1,3-disubstituted
allenes. However, ketones cannot be used as a substrate in
place of aldehydes.
Although significant progress has been made in allene
synthesis based on the catalytic cross-coupling approach, the
reactions hitherto developed still suffer from severe limitations, such as high reaction temperature, high catalyst loading,
expensive catalyst, and relative unstability of the substrates
(such as enamines in Bertrands system). Furthermore, except
in Bertrands system, trisubstituted allenes cannot be synthesized. Consequently, further development of a catalytic
system that can circumvent these limitations is highly
demanded.
Investigations over the past few years have demonstrated
that the palladium-catalyzed cross-coupling of N-tosylhydrazone with a halide is highly efficient, and that the reaction
mechanism involves the generation of Pd carbene and a
subsequent migratory insertion.[11] Very recently, a palladiumcatalyzed three-component coupling of N-tosylhydrazone,
aryl bromide, and terminal alkyne has been developed in our
2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2011, 123, 1146 –1149
Angewandte
Chemie
research group.[11g] On the other hand, we have noted that
Surez and Fu have reported a coupling of terminal alkynes
with diazoesters or diazoamides catalyzed by CuI to afford
3-alkynoates.[12] Notably, cyclopropenation of the alkyne,
which may be expected for such catalytic system,[13] was not
observed. Instead, allene was detected as a minor by-product
(up to 8 % yield). Inspired by this report, and also as an
extension of our recent interest in palladium-catalyzed crosscoupling of N-tosylhydrazone, we have conceived that a novel
method for allene synthesis may be developed based on
copper(I)-catalyzed cross-coupling of N-tosylhydrazone and
terminal alkyne by selecting the appropriate catalytic system.
Herein we report such a copper(I)-catalyzed cross-coupling
reaction, which affords trisubstituted allenes with high
efficiency (Scheme 1 d).
At the outset of this investigation, we employed
N-tosylhydrazone 1 a and phenylacetylene 2 a as the substrates (Table 1). After some initial experiments (Table 1,
Table 1: Optimization of reaction conditions.[a]
Entry
Base/Solvent
CuX (mol %)/Ligand (mol %)
Yield [%][b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15[c]
16[d]
17
18
19
LiOtBu/toluene
LiOtBu/DCE
K2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
Cs2CO3/dioxane
CuI(5)/none
CuI(5)/none
CuI(5)/none
CuI(5)/none
Cu(MeCN)4PF6(5)/none
Cu(MeCN)4PF6(5)/A(6)
Cu(MeCN)4PF6(5)/B(6)
Cu(MeCN)4PF6(5)/C(6)
Cu(MeCN)4PF6(5)/D(6)
Cu(MeCN)4PF6(5)/E(6)
Cu(MeCN)4PF6(5)/F(6)
Cu(MeCN)4PF6(5)/G(6)
Cu(MeCN)4PF6(5)/H(6)
Cu(MeCN)4PF6(5)/I(6)
Cu(MeCN)4PF6(5)/I(6)
Cu(MeCN)4PF6(5)/I(6)
catalyst a (5)
catalyst b (5)
none
trace
7
15
21
trace
37
35
21
23
27
42
37
39
55
73
87
21
27
none
[a] All the reactions were carried out with 1 a (0.50 mmol), 2 a
(0.50 mmol) in 3.0 mL of dioxane for 3 h if not otherwise indicated.
[b] Yield of the isolated product. [c] 5.0 mL of dioxane as solvent. [d] The
ratio of 1 a/2 a is 1.3:1.0 and in 5.0 mL of dioxane.
entries 1–5), we found that in the presence of Cs2CO3 the
coupling of 1 a and 2 a catalyzed by CuI could afford allene 3 a
in 21 % yield (Table 1, entry 4). Encouraged by these initial
results, we proceeded to optimize the reaction. We were
delighted to find that a combination of CuI and ligand could
significantly affect the reaction (Table 1, entries 5–18). Thus, a
series of nitrogen ligands was examined, and it was identified
that the combination of ligand I and Cu(MeCN)4PF6 provided
the optimal result (Table 1, entry 14). We then went on to
screen other reaction parameters and observed that the
reaction was subjected to notable concentration effects. After
careful experimentation, it was concluded that the reaction
carried out with 1 a at a concentration of 0.1m provided the
optimal yield (Table 1, entry 15). Furthermore, the ratio of
substrates was examined, and it was found that a 1 a/2 a ratio
of 1.3:1.0 led to the highest yield of 3 a (Table 1, entry 16).
Notably, without ligand the coupling product could only be
identified in trace amount (Table 1, entry 5). Finally, a control
experiment showed that in the absence of copper catalysts
product 3 a was not detected under the otherwise identical
reaction conditions (Table 1, entry 19).
With the optimized reaction conditions in hand, the scope
of this transformation was studied by using various terminal
alkynes and N-tosylhydrazones. Treatment of N-tosylhydrazone 1 a with a series of terminal alkynes 2 a–l furnished the
corresponding products 3 a–l in moderate to good yields
(Table 2). The reaction was not significantly affected by the
substituents on the aromatic ring of the terminal alkyne. Both
electron-rich (Table 2, entries 2, 5, and 6) and electrondeficient aryl-substituted alkynes (Table 2, entries 3 and 4)
were effective, although a slightly lower yield was observed
when the substituent was p-CF3 (Table 2, entry 4). Notably,
alkoxyl, acetyl, chloro, and trifluoromethyl groups are all
tolerated under the given reaction conditions. The reaction
also worked well with naphthyl alkynes (Table 2, entries 9 and
Table 2: Reaction scope of terminal alkynes.[a]
Entry
2, R
3, Yield [%][b]
1
2
3
4
5[c]
6
7
8
9
10
11
12[d]
2 a, C6H5
2 b, p-MeC6H4
2 c, p-ClC6H4
2 d, p-CF3C6H4
2 e, p-MeOC6H4
2 f, p-tBuC6H4
2 g, m-CH3C(O)C6H4
2 h, m-MeOC6H4
2 i, 2-naphthyl
2 j, 2-(6-MeO-naphthyl)
2 k, 2-thienyl
2 l, n-C5H11
3 a, 87
3 b, 73
3 c, 62
3 d, 50
3 e, 64
3 f, 87
3 g, 69
3 h, 86
3 i, 75
3 j, 75
3 k, 71
3 l, 54
[a] All the reactions were carried out with N-tosylhydrazones
(0.65 mmol), terminal alkynes (0.50 mmol) in 5.0 mL of dioxane for
5 h if not otherwise indicated. [b] Yield of the isolated product. [c] The
reaction was carried out at 80 8C for 8 h. [d] The reaction was carried out
in 3.0 mL of dioxane for 8 h.
Angew. Chem. 2011, 123, 1146 –1149
2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.de
1147
Zuschriften
10), alkynes bearing a heteroaromatic substituent (Table 2,
entry 11), and an alkyl substituent (Table 2, entry 12).
Next, the reaction scope of N-tosylhydrazone was studied.
The reaction was examined with a series of N-tosylhydrazones
1 b–l, which were treated with phenylacetylene 2 a under the
optimized reaction conditions (Table 3). The cross-coupling
Table 3: Reaction scope of N-tosylhydrazones.[a]
Scheme 2. Mechanistic rationale. L = ligand.
Entry
1, R1, R2
4, Yield [%][b]
1
2
3
4
5
6[c]
7
8[d]
9[d]
10[e]
11
12
1 b, p-MeC6H4, Me
1 c, p-MeOC6H4, Me
1 d, p-ClC6H4, Me
1 e, m-O2NC6H4, Me
1 f, m-MeOC6H4, Me
1 g, o-MeC6H4, Me
1 h, 2-naphthyl, Me
1 i, C6H5, C6H5
1 j, C6H5, H
1 k, C6H5, nPr
1 l, PhCH2CH2, H
1 m, -(CH2)5-
4 b, 75
4 c, 64
4 d, 73
4 e, 40
4 f, 67
4 g, 41
4 h, 76
4 i, 40
4 j, 50
4 k, 69
4 l, 35
4 m, 21
[a] All the reactions were carried out with N-tosylhydrazones
(0.65 mmol), terminal alkynes (0.50 mmol) in 5.0 mL of dioxane for
5 h if not otherwise indicated. [b] Yield of the isolated product. [c] The
reaction was carried out for 8 h. [d] The reaction was carried out in
3.0 mL of dioxane with 5 equiv of Cs2CO3 for 3 h. [e] The reaction was
carried out for 11 h.
worked smoothly with N-tosylhydrazone substrates that were
easily derived from aryl alkyl ketones (Table 3, entries 1–7,
10), diaryl ketones (Table 3, entry 8), and aryl aldehydes
(Table 3, entry 9), thus leading to the formation of trisubstituted or disubstituted allene products. However, when R1 is
an aryl group bearing an electron-withdrawing substituent
(Table 3, entry 4), the yield is markedly diminished. Besides,
the coupling with ortho-substituted aryl alkyl tosylhydrazone
took place over a longer reaction time (Table 3, entry 6).
Finally, the reaction also worked for the N-tosylhydrazone
derived from aliphatic aldehyde and ketones, albeit in low
yields (Table 3, entries 11 and 12).
Based on our understanding of the palladium-catalyzed
cross-coupling reaction of N-tosylhydrazones,[11] we proposed
a plausible mechanism to account for the current copper(I)catalyzed coupling (Scheme 2). In the presence of base and
copper(I) salt, copper acetylide A is formed from phenylacetylene. The reaction of copper acetylide A with diazo
substrate B, which is generated in situ from N-tosylhydrazone
in the presence of a base, leads to the formation of copper–
carbene species C. Migratory insertion of alkynyl group to the
carbenic carbon atom gives intermediate D. The allene
product is formed by protonation of intermediate D, in
conjunction with the regeneration of the CuI catalyst. It is
noteworthy that in this pathway the protonation occurs
regioselectively at a triple bond carbon atom. Alternatively,
if the protonation occurs at the carbon atom attached to
copper, the alkyne product 5 will be formed, which may
1148
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undergo rearrangement to afford the allene product. It is
worth mentioning that in the copper(I)-catalyzed coupling of
terminal alkynes with diazoesters or diazoamides previously
reported by Surez and Fu, 3-alkynoates, which correspond to
5, are the main products.[12] Although the reaction mechanism
has not been mentioned in Fus paper, we conjecture that a
similar migratory insertion of a copper–carbene intermediate
may also be involved, followed by the protonation at the
carbon atom attached to the copper center.[14]
In conclusion, we have developed a novel synthesis of
substituted allenes from terminal alkynes and tosylhydrazones through the copper(I)-catalyzed alkynyl migratory
insertion. This approach can also be viewed as the crosscoupling between the “masked” carbenic carbon atom and
vinylidene.[8] Trisubstituted allenes can be directly synthesized
through a transition-metal-catalyzed cross-coupling of two
fragments. The reaction is operationally simple and the
conditions are mild with low catalyst loading and at moderately high temperature. Since the ligands have a significant
effect on the reaction, asymmetric catalysis should also be
possible. Mechanistically, an unprecedented copper–carbene
migratory insertion process is most likely involved,[11, 15, 16] and
is distinctly different from classic copper(I)-catalyzed reactions of diazo compounds. This may open up new possibility
to incorporate copper-catalyzed coupling reactions with diazo
chemistry.
Experimental Section
Typical procedure for the copper(I)-catalyzed cross-coupling of
N-tosylhydrazones and terminal alkynes: Under a nitrogen atmosphere, ethynylbenzene 2 a (51 mg, 0.5 mmol) was added to a mixture
of Cu(MeCN)4PF6 (9 mg, 0.025 mmol), ligand I (10 mg, 0.030 mmol),
Cs2CO3 (489 mg, 1.5 mmol), and N’-(1-phenylethylidene)tosylhydrazine 1 a (187 mg, 0.65 mmol) in 1,4-dioxane (5 mL). The mixture was
stirred at 90 8C for 5 h and was monitored by TLC. The solvent was
then removed in vacuo to provide a crude mixture, which was purified
by column chromatography on silica gel to afford pure 3 a as a
colorless oil (90 mg, 87 %).
Received: September 14, 2010
Revised: November 9, 2010
Published online: December 23, 2010
.
Keywords: allene synthesis · copper · cross-coupling ·
homogeneous catalysis · N-tosylhydrazones
2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2011, 123, 1146 –1149
Angewandte
Chemie
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Supporting Information.
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